The present invention relates, generally, to a self-contained machine and to a method associated with that machine for cleaning and rinsing work pieces and, more particularly, to an improved system and method for brush cleaning semiconductor wafers in the presence of various solutions and then rinsing the wafers.
Machines for polishing and machines for cleaning work pieces and disks in the electronics industry are generally well known. For example, semiconductor wafers, magnetic disks, and other work pieces often come in the form of flat, substantially planar, circular disks. In the manufacture of integrated circuits, semiconductor wafer disks are sliced from a silicon ingot and prepared for further processing. After each wafer is sliced from the ingot, it must be thoroughly polished and then cleaned, rinsed, and dried to remove debris from the surface of the wafer. Thereafter, a series of steps are performed on the wafer to build the integrated circuits in and near the wafer surface, usually including the steps of depositing or otherwise forming one or more dielectric or metal layers overlying the wafer surface. Typically, after the layers are formed on the wafer surfaces, the wafers must be planarized to remove excess material and imperfections.
After each processing step, it is often desirable to thoroughly clean, rinse, and dry the wafers to ensure that debris is removed from the wafers. Thus, a method and apparatus for quickly and efficiently cleaning, rinsing, and drying wafers is needed which facilitates high wafer throughput, while at the same time thoroughly cleaning and drying the wafers with a minimum of wafer breakage. For a discussion of existing wafer cleaning machines, see, for example, Lutz, U.S. Pat. No. 5,442,828, issued Aug. 22, 1995; Frank et al., U.S. Pat. No. 5,213,451, issued May 25, 1993; and Onodera, U.S. Pat. No. 5,357,645, issued Oct. 25, 1994.
Presently chemical-mechanical polishing and/or planarization (CMP), one method for planarizing wafers, is performed by one machine and wafer cleaning and drying is performed by another, separate machine. After a processing layer (i.e., oxide, tungsten or the like) has been formed on the surface of the wafers, the dry wafers are planarized in a CMP polishing machine. The CMP machine planarizes the wafers by removing excess material, and then typically the wafers are rinsed and placed into a wet cassette. After polishing, residual particles still reside on the wafer""s surface. If these particles dry on the wafer prior to cleaning, the microelectronic structures on the wafer may be corrupted. Therefore, it is extremely important to keep the wafers wet prior to final cleaning and drying of the wafers. From the CMP machine the wet cassette is hand carried to a separate post-CMP wafer cleaning and drying machine that is typically located somewhere near the CMP machine.
This conventional practice of utilizing separate machines for wafer polishing and for wafer cleaning and drying has serious drawbacks. First, wafer manufacturers must have personnel, equipment and facilities on hand to transport wafers in a wet environment from a CMP machine to a cleaning and drying machine. Second, having separate machines for polishing wafers and for cleaning wafers consumes a significant amount of clean room space which, as one skilled in the art will appreciate, is very expensive. Third, having two separate operating systems requires operators to become familiar with two different machines and two different sets of controls.
In addition, recent wafer cleaning techniques have incorporated the use of a certain chemical cleaning agents that are difficult and/or dangerous to handle, such as highly diluted hydrofluoric acid (HF) solution, for improved cleaning of semiconductor wafers, particularly for cleaning metallic ion residue from CMP planarization of the wafer surface. HF poses substantial health and safety risks, and thus great care must be taken when designing equipment that utilizes HF to ensure against undesired escape of HF contaminated fluid or fumes. Past HF cleaning operations have been designed around the conventional dip-tank style of wafer cleaning, wherein wafers are held vertically and are immersed in a tank into which fluids, including HF, may be introduced. In such dip tank style cleaners the motion of the wafer through the fluid, the reaction of the fluid with the wafer surface, and circulation of the fluid in the tank are relied upon to clean the wafer surface.
Conventional brush box style cleaners have also been modified to accommodate the use of HF solutions in wafer cleaning. Known HF brush box cleaners introduce HF solution onto the wafer or the brushes of the brush box so that the wafer is then mechanically scrubbed in the HF solution. Because of the extremely hazardous and corrosive nature of the HF, very dilute solutions are preferable for cleaning. As a result of using dilute solutions, the cleaning time is increased. To provide for adequate cleaning and particle removal, past HF cleaners have had to either add additional brush boxes to increase HF scrubbing time, or have had to increase the wafer residence time in the brush box. One way to increase the residence time is to move the wafer through a single brush box at a reduced rate. A problem with slowing the rate at which the wafer moves through the brush box is that it becomes difficult to control the wafer feed rate when the rate becomes too low. The residence time can also be increased by rotating the wafers within the brush box. Unfortunately this adds complexity to the equipment and results in nonuniformity in the cleaned wafer surface. Adding additional brush boxes or lengthening existing boxes is also often unsatisfactory because either solution requires additional scrubbing length which, in turn, increases the overall tool footprint. The added scrubbing length also increases the amount of chemicals used in the process. Thus a need exists for an improved brush box for cleaning semiconductor wafers or other work pieces with chemical cleaning agents such as HF that safely and efficiently scrubs the work piece in the presence of the chemical cleaning agents without overly slowing the work piece feed rate, or increasing tool footprint and with minimum chemical consumption. Ideally such a brush box and cleaning method could be incorporated into an integrated apparatus and method for CMP polishing, cleaning, rinsing, and drying of work pieces.
In accordance with one embodiment of the invention, cleaning apparatus preferably comprises a water track, cleaning stations, and a plurality of work piece staging areas. In a further embodiment of the invention, the cleaning apparatus additionally includes a rinse station and a spin dryer station. In accordance with a preferred embodiment of the invention, the cleaning apparatus comprises an input water track station, a first scrub module, a mid-station water track, and a second scrub module. Each scrub module preferably includes a brush box and a rinse station and is chemically isolated from the other scrub module. Potentially hazardous chemical cleaning fluids are contained within each module using primary and secondary containment, and chemical fumes are exhausted from each module independently.
In accordance with a further embodiment of the invention, the cleaning apparatus is combined with a planarization apparatus to form a single, integrated machine for the planarization and cleaning of work pieces. More specifically, in accordance with a preferred embodiment, a work piece is planarized in a CMP station and then is directly transferred to a cleaning apparatus of the machine. When a work piece is first loaded into the cleaning apparatus from a CMP station of the machine, the work piece is held at a first input water track staging area until a determination is made that the next processing area, the first scrub module, is clear and is ready to receive the work piece. Only then is the work piece released to that next processing module. When cleared, water jets of the water track urge the work piece into the first brush box cleaning station (within the first scrub module) that is configured to scrub and clean both surfaces of the work piece in the presence of a first chemical cleaning agent that, in a preferred embodiment, may be a dilute ammonium hydroxide solution. From the first brush box cleaning station, the work piece is transported into a rinse station which is the second staging area within the first scrub module. The work piece is held at this stage until the work piece is rinsed and a determination is made that the preceding work piece has cleared the next stage area. While held in the rinse station/second stage, water jets spray the top and bottom surface of the work piece to rinse away the first chemical cleaning agent remaining on the work piece before moving the work piece into a mid station that is free of chemical cleaning agents and where the work piece is again staged. From the second stage area, water jets urge the work piece into and past the mid station and then into a second scrub module for a second cleaning of the work piece, using a second chemical cleaning agent that is preferably an HF cleaning solution. Isolation slots in the water tracks and water track shields restrict the first chemical cleaning agent to the first scrub module and the second chemical cleaning agent to the second scrub module. After being scrubbed and cleaned with the second chemical cleaning agent, the work piece exits the second brush box station and is conveyed into the second rinse station/third stage area where the work piece is again rinsed, top and bottom, to remove residual contamination and to neutralize the second chemical cleaning agent. From the third stage area, the work piece is transported via a water track to an additional rinse station. After the additional rinsing, the work piece is moved to a spin dryer station where it is again rinsed and then spun dried. The cleaned work piece is then returned to a cassette.
The brush boxes of the cleaning apparatus preferably comprise a plurality of pairs of brush rollers that pull the work pieces through the brush boxes and that also clean the substantially planar top and bottom surfaces of the work pieces. Various rollers within the brush boxes may operate at different rotational speeds and rotate in different directions. In this manner, certain rollers may function as drive rollers to move work pieces through the cleaning stations, while other rollers may function to clean work piece surfaces as the work pieces are driven through the cleaning stations.
In a particularly preferred embodiment, the rollers are contained in enclosed boxes that may be easily removed from the machine to facilitate convenient changing of the rollers as the roller surfaces become worn through extended use. A plurality of channels are preferably formed in an upper inside surface of the brush boxes to permit distribution of a plurality of different chemicals (e.g., water, cleaning solutions including acidic solutions such as dilute HF solutions and basic solutions such as ammonium hydroxide solutions, surfactants, agents to control the pH of the various solutions, and the like) into discrete regions of the brush boxes. In this manner, work pieces passing through a brush box may be exposed to a first chemical solution in a first stage of the brush box and later exposed to a second chemical solution in a latter stage of the brush box. Since a plurality of brush boxes are preferably employed, different chemicals may be used in each of the different brush box cleaning stations. The first brush box, for example, may distribute a cleaning solution and deionized water mixture onto the work pieces to facilitate heavier cleaning, while the second brush box may distribute a second cleaning solution or may simply distribute deionized water onto the work pieces to achieve a rinse. Each brush box also incorporates sensors positioned proximate the entrance and the exit of the brush box that sense the presence of a work piece and transmit that information to a brush box controller. The controller utilizes the signals from the sensors to adjust the operation of the brush box rollers, for example, to reverse the direction of the drive rollers causing the work piece to reverse direction. In this way, the sensors and controller may be used to cause the work piece to oscillate back and forth within the brush box to thereby maximize the scrubbing time relative to the time spent entering and exiting the brush box.
In accordance with one embodiment of the invention, work pieces are transported from the second scrub module to a rinse station via a water track. Work pieces are rinsed in a serial manner within the rinse station, and then are transferred to a spin/rinse dryer station. The spin/rinse dryer station provides a final rinse of the work piece and then spins the work piece to remove any residual water from the work piece. Work pieces are retrieved from the spin/rinse dryer station and are preferably returned to the cassette from which they originated. Thus each work piece can be tracked and monitored through the polishing and cleaning steps so that after processing, it can be placed back into its original slot in its original cassette.
A vision system or other position sensing method may be utilized to monitor work pieces as they pass through the cleaning apparatus to determine whether the work pieces have properly moved from one area of the apparatus to the next. Based on the monitored information, work pieces are released from the various staging areas only when it is determined that the workpieces are all safely positioned within the proper staging areas; that is, when it has determined that lodged work pieces are not still in the water track or in the various scrubbing, rinsing and drying stations.